The present invention relates to apparatus and circuitry for producing and processing signals from high-temperature environment sensors, and more particularly, to a method and apparatus for obtaining high quality low-noise velocity and/or acceleration parameter signals from a piezoelectric vibration sensor element used in high-temperature, electrically-noisy environments.
Piezoelectric element sensors are commonly used on industrial machinery and equipment to monitor and detect operating conditions of the equipment to provide early warning and detection of malfunctions and/or failure. One known use is to function as an accelerometer for measuring the vibrations of rotating machines such as gas turbines or jet engines. In applications such as these, environmental conditions at the point of monitoring can be quite harsh. For example, monitoring conditions at various points or parts of a high-power gas turbine power generator or within a nuclear power plant reactor may require monitoring under extremely high-temperature and electrically noisy conditions. In the past, to obtain measurements on gas turbines under such conditions, a piezoelectric/piezo-ceramic sensor used at the point of measurement was mounted in a housing having an integral electrical connector to which was connected a lengthy cable conductor to transfer the signal to remote signal processing electronics located away from the harsh conditions at the sensor. However, moisture and vibration can compromise the signal path from the piezoelectric sensor to the signal processing circuiting creating false vibration data and causing the machine to be unnecessarily shut down. Moreover, during a full-power shut-down, the temperature inside the machine enclosure may exceed the maximum temperature rating of the signal processing electronics requiring additional repair and replacement of components. Some major disadvantages of past arrangements included the ohmic connectors between the sensor and electronics and the limited operational temperature range of the processing electronics, which render the assembly susceptible to noise, false data/signal readings and increased machine downtime.
Typically, the signal conditioning and processing electronics used for piezoelectric sensors includes a charge converter or charge amplifier for detecting and amplifying the small electrical charge or changes in charge developed on the piezoelectric element and a gain amplifier for conditioning the signal and producing a usable output signal indicative of, for example, a vibration parameter such as acceleration. It is conventional to use a differential amplifier, called an operational amplifier (OP AMP), having a very high gain and a very high input impedance for the charge amplifier and gain amplifier stages.
One of the problems with obtaining accurate and reliable signals from a piezoelectric sensor placed in a gas turbine or jet engine is that it is subject to extreme temperatures and electrical noise. The high-temperatures makes it nearly impossible to locate the sensitive signal processing circuitry anywhere near to the sensor head, and the noisy electrical environment makes any long electrical signal cable/conductor connections extremely susceptible to picking up unwarranted electrical noise signals which may alter or interfere the relatively weak signal generated by the piezoelectric sensor element.
Another problem is that the differential amplifiers typically used in such applications are susceptible to unequal common-mode noise signals present at the inputs. A difference in common-mode noise signal strength at each input of the differential amplifier causes unwanted inaccuracies in the output signal. Moreover, the degree of unequal common-mode noise observed is specific to the particular design of the sensor assembly. In particular, a major cause of imbalances in common-mode noise at the differential amplifier inputs is primarily a function of parasitic capacitances within the piezoelectric element stack housing, the sensor-to-processing circuit cable, the processing electronics housing, and from other portions of the sensor assembly.